Noninvasive Trauma Monitors
Patients with severe injuries or serious infections run the risk of circulatory shock–a life-threatening condition in which the blood can’t supply tissues with enough oxygen and nutrients. If shock is recognized in time, the patient can be resuscitated with oxygen, intravenous fluids, and medications. But catching shock early is no simple matter. A small infrared sensor currently under development at the University of Massachusetts Medical School promises to detect impending shock earlier than any other noninvasive test.
Traditionally, patients in critical condition are continuously monitored for changes in blood pressure, heart rate, and pulse oxygen saturation. But the body has mechanisms to compensate for massive blood loss and systemic infection, keeping those parameters steady even while the patient’s status deteriorates. “When the blood pressure starts to drop, it’s too late,” says spectroscopist Babs Soller, who developed the new device along with colleagues at the UMass Medical School. “The patient is already going into shock.” The new device instead measures the levels of oxygen, pH, and hematocrit–the proportion of red blood cells in the blood–in a patient’s muscle tissue.
“Until now, we’ve either had noninvasive methods which are very insensitive, like blood pressure, or we’ve had sensitive methods that are invasive and cumbersome,” says George Velmahos, chief of trauma, emergency surgery, and surgical critical care at Massachusetts General Hospital, who was not involved in developing the device. “So the noninvasive and continuous nature of this method is key.”
Soller’s device beams near-infrared light through the skin over an arm or leg muscle, where it travels through fat and reflects off muscle tissue and back to the monitor. Based on the spectrum of the reflected light, computer algorithms determine the oxygen, pH, and hematocrit levels. Unlike similar infrared biomeasurement devices, the new monitor automatically compensates for differences in skin color and fat thickness between patients to optimize the results.
One of the ways that the body compensates for blood loss or impaired circulation is by prioritizing which tissues most need oxygen. Blood is shunted away from skeletal muscles and internal organs and delivered instead to the heart and brain. A pulse oximeter, which analyzes the blood before it has delivered oxygen to tissues, can’t tell whether this kind of compensation is occurring. Because the new device measures oxygen within the muscle, it can give a more complete picture of how well the blood is feeding peripheral tissues. A substantial drop in muscle oxygen while pulse oxygen saturation remains steady could indicate that the patient is compensating for internal bleeding and will soon “crash.”
Testing blood pulled from vessels near the heart can provide even more information, but that requires a painful, highly invasive, labor-intensive procedure. And since blood samples must be sent to a laboratory for analysis, results are often too delayed to be useful in treating an unstable patient. Soller’s monitor, in contrast, noninvasively provides continuous, real-time results.
While other devices, such as Hutchinson Technology’s InSpectra StO2 Tissue Oxygenation Monitor, can noninvasively measure tissue oxygenation, Soller’s is the only one that measures pH and hematocrit levels as well. Muscle pH is an important indicator of how well treatment is working, says Soller. Cells deprived of oxygen cease to function properly, leading to acid buildup, which lowers pH and causes further damage. When the patient is resuscitated, tissue oxygen levels are restored before tissue pH recovers. Without a good measure of tissue pH, doctors have no way to know whether the patient needs further resuscitation.
Measuring the hematocrit level adds yet another dimension. Administered fluids may rescue a trauma patient’s plummeting blood pressure, but they can also dilute the blood, reducing the proportion of red blood cells and impairing oxygen delivery. A falling hematocrit level would alert doctors to this problem early enough to properly address it.
Soller and her colleagues have been working on their monitor for more than a decade. In October, at the 2008 Center for Integration of Medicine and Innovative Technology Congress, in Boston, they exhibited a new, fully portable version of the device that’s under a pound, less than one-eighth its previous weight.
The device, whose development is funded in part by the army, could potentially be used by combat medics to predict shock in critically wounded patients, and to monitor patients who appear stable for reactions to undiagnosed internal injuries. Soller hopes that, beyond aiding in military applications, the monitor will be useful in civilian ambulances, emergency rooms, and intensive-care units, and in the operating room for surgeries associated with high blood loss.
Because the device is noninvasive, it has been through extensive preliminary testing. In one study, patients had the lower halves of their bodies strapped into a vacuum chamber, which sucked blood toward their feet to mimic massive blood loss. The monitor was able to detect changes before the patients’ blood pressure and pulse oximeter readings began to drop, a promising sign that it could serve as an effective early-warning system.
Before the device can be used in clinical settings it will need to receive U.S. Food and Drug Administration approval. A company called Reflectance Medical has been founded to help guide the monitor to market. Meanwhile, Hutchinson’s InSpectra monitor, which has been commercially available since 2007, is already in clinical use.
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